Just finished Morbidly Curious. Absolutely held up from start to finish. Utterly fascinating book. If you're even remotely interested in or curious about horror, read this book. It's a quick read. Only 272 pages in print and it took me a little more than 5 hours to read as an ebook. It just took me awhile to put in that time because of the ADHD and constant distractions.
To the question of why some people are fans of horror. Read the book. About 70% of the text is providing various answers to that exact question. The short answer is: everything. Biology, psychology, evolution, society, etc. It's more accurate to say that a few people somehow aren't fans of horror or that some adults have grown out of their morbid curiosity.
There's also so much mythbusting about horror fans here that anyone interested in the genre should pick it up just to have as a reference to push back against all the negative stereotypes about horror fans. The author is a PhD psychologist who literally did the research. And he also extensively cites his sources. The last 30% of the book is notes with links to various papers used in the writing of the book. It's also well indexed.
And, while not directly related, it also offered some great insights into humans generally and why some things work the way they do. Why are bosses like that? Why does bad news travel faster? Etc.
If you're a horror writer or a horror referee this is also worth the read as it will offer some insights. It's not a how-to book by any stretch, but there's definitely stuff here worth mining.
I don't want to oversell the thing, but it's easily the best non-fiction book I've read in last few years.